Why did Portia say, "Your wife would give you little thanks, if she were present, to hear you make this offer"? What was the offer, by whom?

In Act IV, Scene 1 of The Merchant of Venice, the trial has been held and Shylock has won and sentence is just about to be pronounced, so Antonio has the opportunity to speak in his behalf and says that he will face the judgement against him and be spared the misery of outliving his fortune and being penniless in old age. He also tells Bassanio not to grieve that he is paying Bassanio's...

In Act IV, Scene 1 of The Merchant of Venice, the trial has been held and Shylock has won and sentence is just about to be pronounced, so Antonio has the opportunity to speak in his behalf and says that he will face the judgement against him and be spared the misery of outliving his fortune and being penniless in old age. He also tells Bassanio not to grieve that he is paying Bassanio's debt with his life. Bassanio responds to Antonio's speech by saying that even though he love his wife as much as life itself, he would sacrifice her to the court if it meant he could save Antonio's life, which he values and loves even more than his dear wife's life.

Of course, since Portia in disguise is Bassanio's wife, she is not altogether charmed to hear Bassanio say he would trade her life to safeguard Antonio's, and admonishes Bassanio that his wife wouldn't thank him for offering (suggesting) to sacrifice her life in exchange for Antonio's safety. Gratiano and Nerissa have a repeat of the same essential conversation when Gratiano declares that he wishes his wife were already dead and in heaven to intercede and beseech the help of heaven in saving Antonio. Of course, Nerissa wasn't all that happy to hear that Gratiano could wish her dead already.